They eventually sent a replacement camera (no cost) after I told them I tried the camera on a different router and network in the state where I live. “Thumbnail failed” kept being the result. I had to send the problem camera back to them in the box they sent the new camera in.
But first they said, since the Jetpack can accommodate up to 10 devices, “What you can do is to check if there are 10 devices already connected to the Verizon Jet Pack, if its not the case you can also try to reboot the camera first then reboot the Verizon Jet Pack in the same room as the camera and the sync module and see how it goes.”
I brought the camera back home with me to try it on my more powerful home wifi (150 MBPS), which is with the cable company, Comcast. Same problem. The Blink tech said that was a good way to test, then started the replacement process.
The distance between the module, router and camera have a lot to do with it, as does, I believe, the type of construction material separating them. In the ranch house where this was a problem, a double brick chimney separated the camera from the router.